Disclosed herein are ultraviolet (UV) curable silsesquioxane-containing write-through optical fiber coatings for fabrication of optical fiber Bragg gratings and the fibers made therefrom.
Fiber Bragg grating (FBG) arrays are seeing an increased demand for a variety of sensing applications. Improvements are needed with respect to a) coating heat resistance in emerging markets, e.g., geophysical applications (particularly for temperature and strain sensing in oil exploration) and b) FBG production efficiency, especially for shape sensor fibers that are produced by single-shot (single laser pulse) writing techniques. FBG fabrication using a “strip/write/recoat” process is not efficient for high-volume arrays, being time consuming and also subjecting the stripped fiber to an increased probability of breakage thereby reducing yields and manufacturing efficiency. In such a process, the polymer coating is removed from the glass substrate of the fiber, followed by laser inscription of the FBG into the glass core, and subsequent recoating to restore mechanical robustness afforded by a protective polymer layer. In those cases, removal of the coating may be necessitated by the relative opacity of certain types of optical fiber coatings to the FBG writing wavelength. Draw tower fabrication (writing FBGs during fiber drawing, before the glass fiber is coated) suffers from difficulties involving decoupling the writing process from the fiber drawing process.
Previously, certain “write-through” coating methods were devised that allow inscription of gratings by side-writing with UV lasers through the fiber coating. This was accomplished by tailoring the coating to have increased UV transparency at the writing wavelength. However, such coatings to date have either not been UV-curable (presenting difficulties with solvent removal or thermal curing) or those that are UV-curable have provided only modest levels of robustness with respect to scuff resistance and thermal stability.
It has been found that fabrication of certain types of FBGs is particularly sensitive to the surface quality of the coating, as for example in shape sensor fiber (SSF) arrays where the fibers contain multiple twisted glass cores and where each grating is inscribed simultaneously into the multiple cores using only a single pulse. Such a single-pulse process is in contrast to FBG inscription processes that use a series of laser pulses or steady UV irradiation, where the FBG quality and intensity can optionally be monitored and adjusted using real-time feedback over a timeframe of multiple seconds or even minutes. Other previously developed UV-curable coatings have shown susceptibility to scratching, wrinkling, or to attraction to dust and environmental debris onto the coating surface, by virtue of having limited hardness and/or excess surface tack associated with relatively low cure speed. Improved write-through coatings are needed with respect to these attributes, particularly for producing single-pulse FBGs.